Characterisation of the solar corona, the interplanetary plasma and the Io plasma torus with radio links to deep space probes

Radio-tracking links are used for many scientific experiments (e.g. geodesy, fundamental physics, and atmospheric sciences). In these types of analyses accurate measurements are needed. In many cases ionospheric and interplanetary plasma represents an important source of noise that must be isolated from the non-dispersive contributions. The effects of charged particles on radio signals are well known and have been extensively exploited for probing the Earth ionosphere, the interplanetary plasma and the solar corona. In this thesis I analyse the effects of plasmas on the radio-metric measurements of planetary spacecraft (ESA’s BepiColombo, NASA’s Juno and JAXA’s Akatsuki) to characterise the properties of the region crossed by the radio signal. In the first part, I use Doppler and range measurements from the BepiColombo’s superior solar conjunction experiments: in this configuration the Earth, the Sun and the probe are almost aligned with the Sun in between, so the ray path crosses the solar corona, offering an opportunity to investigate its properties. These experiments allow to sample solar plasma and solar wind in the acceleration region of the Sun where only a few measurements are available. The data set spans over different phases of the solar cycle, making the analysis particularly valuable. The turbulent regime of the solar wind is characterised through the computation of spectra from both closed-loop and open-loop data; in particular, the solar plasma velocity and the inner scale of turbulence are estimated, and the power-law index is compared with the expected Kolmogorov value (indicating well-developed turbulence). A cross-correlation method to precisely localise in space and time plasma structures (like coronal mass ejections) crossing the line of sight is then presented. Range measurements are used to compute the total electron content and compare electron density models for the solar plasma. Doppler measurements are exploited to identify quasi-periodic components indicative of magneto-acoustic waves that may have a role in the solar wind acceleration. Finally, I present a correlative analysis between BepiColombo and Akatsuki data sets to estimate the solar wind velocity. In the second part of the thesis, Juno Doppler measurements are exploited to identify a suitable electron density model to describe the Io plasma torus (IPT). The reference model divides the torus in three regions plus an extended torus, and both single-arc and global approaches are employed. I show that an axisymmetric model represents only an average behaviour of the torus over the observation period and is not able to represent correctly all the analysed passes. Time and longitude variations are added to the model, leading to an improvement of the results. The IPT has been probed by only a few space missions. While Juno’s data set is quite valuable, the variability of the IPT density makes any modelling a complex task. To this end, every future opportunity to probe the IPT would be beneficial. The JUICE mission, arriving at the Jupiter system in 2031 and orbiting the planet and Ganymede for more than four years, may contribute very significantly to the investigation of the IPT. In this work I identify the best opportunities for radio occultations of the Io torus, with the goal of enhancing our knowledge of its structure and its role in the Jupiter’s magnetosphere.